System and method for steganographic image display

ABSTRACT

A computer-implemented method for generating images. The method includes receiving first and second target images. The method further includes computing a delta image based on a difference between the first target image and the second target image and a technique for multiplexing a first display image with the delta image, where the first display image multiplexed with the delta image, when viewed by a person in an ambient setting, is perceived as the second target image. Advantageously, a hidden image is obscured from an ambient observer, while still providing the ambient observer with a target image that is intended to be perceived.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/386,896, filed Apr. 24, 2009, which is hereby incorporatedherein by reference.

BACKGROUND

1. Field of the Invention

The present invention relates to the field of computer graphics and, inparticular, to a system and method for steganographic image display.

2. Description of the Related Art

Steganography is the science of obscuring a desired message or imagefrom a casual observer so that, through the use of the correct key, anobserver can un-obscure the desired message or image. One typicalexample of using steganography to obscure an image involves cereal boxhidden message decoders. A cereal box may include a piece of coloredplastic that, when placed over an obscured image printed on the cerealbox, reveals the hidden image. The image that is printed on the cerealbox is typically a muddled image that does not portray a desired orvaluable image. For example, the image that is printed on the cereal boxmay be a “noise field” that is meant to hide the hidden image.

One problem with conventional steganographic techniques for obscuringhidden images is that often the observer can detect that a hidden imageis being concealed. In some steganographic techniques, the desired imageis visually present in the image that is observed by the observer, butis at least partially obscured. Typically, the observer can often see a“hint” of the image, even by viewing without any particular decoder.This creates a puzzle-breaking psychology, where the observer knows thatan image is being hidden and seeks to detect the hidden image withoutany decoder.

A second problem is that the image viewed by the observer without adecoder is not an interesting image. As described, often the hiddenimage is obscured by a noise field. The noise field added to the hiddenimage creates an undesirable and uninteresting image.

As the foregoing illustrates, there is a need in the art for an improvedtechnique for steganographic image display.

SUMMARY

Embodiments of the invention provide a system and method for generatingimages. A person that views a display image without looking though aspecialized filter perceives a first target image that is intended to beperceived. When the person views the display image through a specializedfilter, then the person perceives a second target image. Embodiments ofthe invention are achieved by computing a delta image that is related tothe difference between the two non-noise images, and multiplexing thesecond non-noise image with the delta image.

One embodiment of the invention provides a computer-implemented methodfor generating images. The method includes receiving first and secondtarget images. The method further includes computing a delta image basedon a difference between the first target image and the second targetimage and a technique for multiplexing a second display image with thedelta image, where the second display image multiplexed with the deltaimage, when viewed by a person in an ambient setting, is perceived asthe first target image.

Another embodiment of the invention provides a system including a deviceconfigured to cause a first display image multiplexed with a delta imageto be displayed, where the second display image multiplexed with thedelta image, when viewed by a person in an ambient setting, is perceivedas a first target image, where the delta image is based on a differencebetween the first target image and the second target image and atechnique for multiplexing the second display image with the deltaimage.

Yet another embodiment of the invention provides a computer-implementedmethod for generating images. The method includes receiving first,second, and third target images. The method further includes computing adelta image based on a difference between the first target image, thesecond target image, and the third-target image, and a technique formultiplexing the delta image with a second display image and a thirddisplay image, where the delta image multiplexed with the second displayimage and the third display image, when viewed by a person in an ambientsetting, is perceived as the first target image.

Advantageously, embodiments of the invention provide a technique forobscuring an image from an ambient observer, while still providing theambient observer with a target image that is intended to be perceived.Thus, the ambient observer would not be able to detect that an image isbeing hidden within the displayed image or obscured by the displayedimage that the ambient observer perceives.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the inventioncan be understood in detail, a more particular description of theinvention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a system configured to implement one ormore aspects of the present invention.

FIG. 2 is a conceptual diagram illustrating two images configured forsteganographic image display and creation of a third image, according toone embodiment of the invention.

FIG. 3A is a conceptual diagram of a display image viewed by twoviewers, according to one embodiment of the invention.

FIG. 3B is a conceptual diagram of a system for steganographic displayimage display, according to one embodiment of the invention.

FIG. 4A is a conceptual diagram illustrating a system where timedivision multiplexing is used to display the display image, according toone embodiment of the invention.

FIG. 4B is a conceptual diagram illustrating three images configured forsteganographic image display and creation of a fourth image, accordingto one embodiment of the invention.

FIG. 5 is a conceptual diagram illustrating three images configured forsteganographic image display of a fourth image, according to oneembodiment of the invention.

FIG. 6 is a conceptual diagram of a display image viewed by threeviewers, according to one embodiment of the invention.

FIG. 7 is a flow diagram of method steps for generating a target imagethat includes a sub-image that is also a target image, according to oneembodiment of the invention.

FIG. 8 is a flow diagram illustrating how using a filter causes a viewerto perceive the target image differently, according to one embodiment ofthe invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention provide a system and method for imagedisplay. A person that views a display image without looking though aspecialized filter perceives a first non-noise target image that isintended to be viewed. When the person views the display image through aspecialized filter, then the person perceives a second non-noise targetimage. Embodiments of the invention are achieved by computing a deltaimage that is the difference between the two non-noise images, andmultiplexing the second non-noise image with the delta image.

FIG. 1 is a block diagram of a system 100 configured to implement one ormore aspects of the present invention. System 100 may be a digital videoplayback device, computer workstation, personal computer, video gameconsole, personal digital assistant, rendering engine, or any otherdevice suitable for practicing one or more embodiments of the presentinvention.

As shown, system 100 includes a central processing unit (CPU) 102 and asystem memory 104 communicating via a bus path that may include a memorybridge 105. CPU 102 includes one or more processing cores, and, inoperation, CPU 102 is the master processor of system 100, controllingand coordinating operations of other system components. System memory104 stores software applications and data for use by CPU 102. CPU 102runs software applications and optionally an operating system. Memorybridge 105 is connected via a bus or other communication path to an I/O(input/output) bridge 107. I/O bridge 107 receives user input from oneor more user input devices 108 (e.g., keyboard, mouse, joystick,digitizer tablets, touch pads, touch screens, still or video cameras,motion sensors, and/or microphones) and forwards the input to CPU 102via memory bridge 105. In some embodiments, input/output devices mayinclude a radio frequency identification (RFID) tag and/or RFID reader.Additionally, any other technically feasible technique for implementinguser identification is within the scope of embodiments of the invention.

A display processor 112 is coupled to memory bridge 105 via a bus orother communication path; in one embodiment display processor 112 is agraphics subsystem that includes at least one graphics processing unit(GPU) and graphics memory. Graphics memory includes a display memory(e.g., a frame buffer) used for storing pixel data for each pixel of anoutput image. Graphics memory can be integrated in the same device asthe GPU, connected as a separate device with the GPU, and/or implementedwithin system memory 104.

Display processor 112 periodically delivers pixels to a display device110 (e.g., a screen or conventional CRT, plasma, OLED, SED or LCD basedmonitor or television). Additionally, display processor 112 may outputpixels to film recorders adapted to reproduce computer generated imageson photographic film. Display processor 112 can provide display device110 with an analog or digital signal.

A system disk 114 is also connected to I/O bridge 107 and may beconfigured to store content and applications and data for use by CPU 102and display processor 112. System disk 114 provides non-volatile storagefor applications and data and may include fixed or removable hard diskdrives, flash memory devices, and CD-ROM, DVD-ROM, Blu-ray, HD-DVD, orother magnetic, optical, or solid state storage devices.

A switch 116 provides connections between I/O bridge 107 and othercomponents such as a network adapter 118 and various add-in cards 120and 121. Network adapter 118 allows system 100 to communicate with othersystems via an electronic communications network, and may include wiredor wireless communication over local area networks and wide areanetworks such as the Internet.

Other components (not shown), including USB or other port connections,film recording devices, and the like, may also be connected to I/Obridge 107. For example, an audio processor may be used to generateanalog or digital audio output from instructions and/or data provided byCPU 102, system memory 104, or system disk 114. Communication pathsinterconnecting the various components in FIG. 1 may be implementedusing any suitable bus or point-to-point communication protocol(s), andconnections between different devices may use different protocols, as isknown in the art.

In one embodiment, display processor 112 incorporates circuitryoptimized for graphics and video processing, including, for example,video output circuitry, and constitutes a graphics processing unit(GPU). In another embodiment, display processor 112 incorporatescircuitry optimized for general purpose processing. In yet anotherembodiment, display processor 112 may be integrated with one or moreother system elements, such as the memory bridge 105, CPU 102, and I/Obridge 107 to form a system on chip (SoC). In still further embodiments,display processor 112 is omitted and software executed by CPU 102performs the functions of display processor 112.

Pixel data can be provided to display processor 112 directly from CPU102. In some embodiments of the present invention, instructions and/ordata representing a scene are provided to a render farm or a set ofserver computers, each similar to system 100, via network adapter 118 orsystem disk 114. The render farm generates one or more rendered imagesof the scene using the provided instructions and/or data. These renderedimages may be stored on computer-readable media in a digital format andoptionally returned to system 100 for display. Similarly, stereo imagepairs processed by display processor 112 may be output to other systemsfor display, stored in system disk 114, or stored on computer-readablemedia in a digital format.

Alternatively, CPU 102 provides display processor 112 with data and/orinstructions defining the desired output images, from which displayprocessor 112 generates the pixel data of one or more output images,including characterizing and/or adjusting the offset between stereoimage pairs. The data and/or instructions defining the desired outputimages can be stored in system memory 104 or graphics memory withindisplay processor 112. In an embodiment, display processor 112 includes3D rendering capabilities for generating pixel data for output imagesfrom instructions and data defining the geometry, lighting shading,texturing, motion, and/or camera parameters for a scene. Displayprocessor 112 can further include one or more programmable executionunits capable of executing shader programs, tone mapping programs, andthe like.

It will be appreciated that the system shown herein is illustrative andthat variations and modifications are possible. The connection topology,including the number and arrangement of bridges, may be modified asdesired. For instance, in some embodiments, system memory 104 isconnected to CPU 102 directly rather than through a bridge, and otherdevices communicate with system memory 104 via memory bridge 105 and CPU102. In other alternative topologies display processor 112 is connectedto I/O bridge 107 or directly to CPU 102, rather than to memory bridge105. In still other embodiments, I/O bridge 107 and memory bridge 105might be integrated into a single chip. The particular components shownherein are optional; for instance, any number of add-in cards orperipheral devices might be supported. In some embodiments, switch 116is eliminated, and network adapter 118 and add-in cards 120, 121 connectdirectly to I/O bridge 107.

FIG. 2 is a conceptual diagram illustrating two images 202, 204configured for steganographic image display and the creation of a thirdimage 206, according to one embodiment of the invention. As shown, image202, also referred to herein as a “hidden image,” is composed of aplurality of image locations, where the color at each of the pluralityof image locations is a frequency spectrum referred to as spectrum “H.”Image 204, also referred to herein as a “delta image,” is composed of aplurality of image locations, where the color at each of the pluralityof image locations is a frequency spectrum referred to as spectrum delta“Δ.” Image 206, also referred to herein as an “ambient image,” iscomposed of a plurality of image locations, where the color at each ofthe plurality of image locations is a frequency spectrum referred to asspectrum “A.”

In one embodiment, image 202 and image 206 are input images. The image204 is computed by calculating the difference between the image 202 andthe image 206 at each image location, as illustrated in FIG. 2 andequation 208. As described, the spectrum Δ at a first image location inimage 204 is configured so, that when the spectrum Δ is combined withthe corresponding spectrum H at the first image location in image 202,then the combination equals the corresponding spectrum A at the firstimage location in image 206, as illustrated by equation 210. Similarly,when the spectrum Δ at the first image location is excluded from thespectrum (H+Δ), then the remaining spectrum equals spectrum H at thefirst image location, as illustrated by equation 212.

In some embodiments, both image 202 and image 206 are “target images,”meaning that both images represent an image that is intended to beperceived. Image 204, in those embodiments, would be a non-target image,since image 204 would provide whatever “offset” spectra are necessary tosupply the difference between image 202 and image 206. In oneembodiment, image 202 and image 206 portray similar images, but withsome minor variations. For example, image 206 may portray a scene with avariety of objects, and image 202 may portray the same scene with anadditional “hidden” object present. In alternative embodiments, image202 and image 206 portray apparently unrelated images that appear verydissimilar. For example, image 206 may portray an outdoor nature scene;whereas, image 202 may portray a stock market trading floor.

In some embodiments, image 206 is viewed by an observer that is notlooking through any filter(s). Also, image 202 can be recovered from theimage 206 by application of an additional “not(Δ)” filter that eitherblocks spectrum Δ or selectively passes spectrum H. For example, theimage 206 may be projected by a projector onto a theater screen. Viewersthat are wearing special glasses that provide the not(Δ) filter can seeimage 202, but will not see image 206; whereas, viewers that are notwearing any type of specially-filtered glasses will see image 206. Inthis fashion, two different viewers, one wearing a specialized filterand one not wearing any specialized filter, can see two completelydifferent images when looking at the same screen. In some prior arttechniques, each viewer wears individual specialized headgear and cansee a different image when looking at the same display device. However,in these prior art techniques, an ambient viewer (i.e., a viewer that isnot wearing any specialized or filtering headgear) does not see a“target image.” Rather, in these prior art techniques, the ambientviewer sees a blank image, or significantly “noisy” image that does notportray a useful or desirable image.

Various techniques can be used to generate image 206 from thecombination of images 202 and 204. In one embodiment, two differentprojectors may be used to generate image 206, where a first projectorprojects image 202 and a second projector projects image 204coincidently. As described, in order for a viewer to view the ambientimage 206, no specialized filtering mechanism is needed. However, for aviewer to view the hidden image 202, some filter is required.

In another embodiment, images 202 and 204 are time division multiplexed.As is known, when two different images are displayed in fast succession,a human's eyes and brain cannot distinguish between the two images and,instead, perceives an image that is a combination of the two images. Forexample, if the images 202 and 204 are being provided by a projectorthat projects images, then at time t=1 the projector may display image202 and at time t=2 the projector may display image 204. The projectormay alternate, at each timestep, between image 202 and image 204. Iftime division multiplexing is used to combine images 202 and 204, then atime synchronizing device that exposes only one of the images to theviewer can be used to reveal the hidden image. For example, liquidcrystal display (LCD) shutter glasses may be configured so that bothshutters are transparent at the same time and opaque at the same time.In this manner, each time that image 204 is being displayed, image 204may be blocked by the LCD shutter glasses, and each time that the hiddenimage 202 is displayed, the shutters of the LCD shutter glasses wouldnot block the hidden image 202, and hence the hidden image 202 passes tothe eyes of the viewer. In another example, some other light-blockingobject that can be synchronized in time with the projection of the deltaimage 204 so that the light-blocking object occludes the delta image 204may also be used.

In alternative embodiments, rather than using time divisionmultiplexing, other techniques may be used to combine images 202 and204. In one embodiment, frequency division multiplexing may be used tocombine images 202 and 204. Accordingly, an ambient viewer can view thedisplayed image 206 that contains all frequencies contained in bothimages 202 and 204; whereas, a viewer that is viewing the displayedimage 206 through a filter that does not pass the certain lightfrequencies contained in image 204 perceives the hidden image 202. Inanother embodiment, phase division multiplexing may be used to combineimages 202 and 204. Accordingly, a viewer that is observing thedisplayed image 206 through a polarizing filter that passes light of aparticular phase or phase range can view image 202. In furtherembodiments, one or more of a optical polarization, shuttering, depth offield alteration, light intensity alteration, light angle alteration,anaglyph filters, and/or other filtering techniques may be used tocombine images 202 and 204.

Additionally, in various alternative embodiments, the filter(s) thatblock the delta image 204 need not be worn by viewers in order toperceive the hidden image. In one embodiment, an LCD shutter panel isplaced in a diorama or stage set, so that the hidden image 202 isvisible when the viewer peers through the panel; whereas, the ambientimage 206 is visible when the viewer does not peer through the panel. Inone embodiment, certain windows in a wall may contain spectral frequencyfilters so that viewers peering thorough these windows see the hiddenimage 202, while viewers peering through unfiltered windows see theambient image 206. In another embodiment, a viewer may examine a scenethrough a magnifying glass, telescope, or similar apparatus thatincludes filters that allow the viewer to perceive the hidden image 202.In yet another embodiment, hidden image filters can be combined with thefilters used to present 3D stereoscopic image display so that someviewers see hidden stereoscopic content. In still another embodiment ofthe invention, the images 202 and 204 may be projected onto a wall, suchas a wall within a theme park. For example, persons that look at thewall while wearing specialized filtered glasses may perceive image 202,while persons that look at the wall without a specialized filterperceive image 206. In another embodiment, image 202 is painted orotherwise imbedded onto a wall or projection surface, and image 204 isprojected onto it. As understood by persons having ordinary skill in theart, various other configurations are also within the scope ofembodiments of the invention.

In some embodiments, depending on the precision, tolerance, and/orresolution of the particular filter being used to extract image 204 fromimage 206 (e.g., time, frequency, phase, etc.) and/or the design of theimages 202, 204, the system 200 can be extended such that three or moretarget images may be perceived by different viewers. Additionally,embodiments of the invention work equally for still images, movingimages, and/or printed images.

Additionally, there are additional calibration and/or encoding and/ordelivery and/or configuration adjustments that may used to improve theperception of both target images 202, 206. In order to achieve theresults described herein, the exclusion process that removes image 204from image 206 should be exactly (or almost exactly) the inverse of theprocess that combines images 202 and 204. If these two operators are notwell matched, then hints of the hidden image 202 may be visible in theambient image 206.

In the physical world, there are certain behaviors and imperfectionswith technology for which compensation may be needed. For example,projection equipment, electronics systems, or viewing filters may alterthe visible spectra of either image 202 or image 204, leading to visibleartifacts or leaving hints of the hidden image 202 in image 206. Foreach implementation of embodiments of the invention, a separate analysismay be needed to determine what calibrations are necessary to achievethe desired results. For example, a display system may have a finite setof colors that can be displayed, which is referred to as the gamut ofthe display. If the spectrum H or the spectrum Δ does not occur in thegamut of the display device, alternate colors must be carefully chosenwhich represent the missing colors as closely as possible withoutrevealing the existence of the hidden image. Alternatively, if themultiplexed spectrum H+Δ does not occur in the gamut of the displaydevice, the device will display a color which is similar to H+Δ whichdoes not match spectrum A. In this case, alternate colors for H and Δmust be chosen that are in the display gamut and will match A as closelyas possible without revealing the existence of the hidden image. Due tothese calibrations and adjustments, the computation of the delta image204 will include more computation than merely the difference of thecolors in image 202 and image 206.

FIG. 3A is a conceptual diagram of a display image 324 viewed by twoviewers 310, 312, according to one embodiment of the invention. Asshown, the display image 324 includes image locations 302, 304, 306,308. Each image location may be composed of data from image 202 andimage 204, as described in FIG. 2. For example, the data may be spectraldata. For each image location, the total spectrum is composed of a firstportion that is associated with image 202 (spectrum H) and a secondportion that is associated with image 204 (spectrum Δ). The viewer 310may be viewing the display image 324 in an ambient setting with nofilter. In contrast, the viewer 312 may be viewing the display image 324through a filter 318. The filter 318 may be configured as a “not(Δ)”filter that does not pass frequencies from image 204, e.g., does notpass frequencies from spectrum Δ.

The viewer 310, when viewing the display image 324, perceives view 320.Each image location in view 320 has a spectrum that is the combinationof the spectra from image 202 (spectrum H) and image 204 (spectrum Δ).As described in FIG. 2, the combination of these spectra results inspectrum A. In contrast, viewer 312, when viewing the display image 324through the filter 318, perceives view 322. Each image location in view322 has a spectrum that is the combination of the spectra from image 202and image 204 (spectrum H+Δ=A) less the spectrum from image 204(spectrum Δ). In other words, viewer 312 perceives spectrum (H+Δ)−Δ=H.As described in FIG. 2, the result of these operations is image 322(spectrum H). As also described above, each of images 322 and thedisplay image 324 are “target” images, where both images are desirablewhen perceived. In contrast, image 204 may be a non-target image.

FIG. 3B is a conceptual diagram of a system 350 for steganographicdisplay image display, according to one embodiment of the invention. Asshown, the system includes functional blocks 356, 360, 376 representinga display device, functional blocks 366, 368 representing a compareoperation, functional block 374 representing a multiplexing operation,functional block 380 representing a demultiplexing operation, functionalblock 364 representing a processing operation, images 352, 354, 358,362, 370, 372, 378, 382, a memory 384, and viewers 310, 312.

Images 352 and 354 represent two input images that are both targetimages. Image 352 may be designated as the “hidden” image and image 354may be designated as the “ambient” image. As described herein, a deltaimage 372 may be computed based in part on the difference between theambient image and the target image. When the delta image is displayed asbeing multiplexed with the hidden image, the resultant image isperceived as the ambient image in an ambient setting. In addition, theresultant image is perceived as the hidden image when viewed through afilter/demultiplexor 380 that blocks the delta image.

However, as described above, physical constraints and/or imperfectionsmay be introduced into the system 350 at various points in the workflow.In some embodiments, perceived images are constrained by the limitationsof the display devices. For example, the input image 354 (A), whendisplayed on the display device 356, may be perceived image as image 358(A′), which is slightly different than the input image 354. Similarly,image 352 (H), when viewed via a display device 360, may be perceivedimage as image 362 (H′), which is slightly different than the inputimage 352. In one embodiment, display devices 360 and 356 are the samedisplay device. In alternative embodiments, display devices 360 and 356are different display devices.

In some embodiments, further limitations and/or constraints areintroduced into the system 350 by the processing operation 364. Forexample, the processing operation 364 may perform “rounding-up” or“rounding-down” operations to various values, which may introduceartifacts. Additionally, the processing operation 364 may clamp theoutputs to be within a certain displayable range of a particular displaydevice, which may also introduce artifacts.

In still further embodiments, further limitations and/or constraints areintroduced into the system 350 by the multiplexing operation 374 and/orthe demultiplexing operation 380. As described, operations 374 and 380may not be “perfect” inverses of one another when performed in thephysical world.

In order to overcome these limitations and constraints, as well as otherlimitations that may be understood by those with ordinary skill in theart, processing block 364 may in some embodiments generate the displayimage 370 ( H) whose spectrum will better overcome the limitations ofsystem 350, or will provide images 378 and 382 that are perceptuallycloser to images 358 and 362, or which leave fewer hints of image 382 inimage 378 when perceived by viewer 310. In some embodiments, thedifference between image 362 and 382, and the difference between images358 and 378, can be provided to processing block 364 from comparisonblocks 366 and 368, and this information can be used in the calculationof the delta image 372 and display image 370.

As described, the functional block 364 may comprise processingoperations that generate image 370 and image 372. As described ingreater detail herein, images 370 and 372 may be multiplexed via amultiplexing functional block 374. In one embodiment, image 370 isidentical to image 352. In another embodiment, image 370 is identical toimage 362. In yet another embodiment, image 370 is not identical toeither image 352 or image 362. In these embodiments, image 370 may,however, be perceived as image 352 and/or image 362. For example, thedifference between image 370 and image 352 may be imperceptible to aperson, or the difference may be very slight yet perceptible. The outputof the multiplexor 374 is then displayed on a display device 376. Whenperceived by a viewer 310 that is viewing the output of the displaydevice 376 in an ambient setting without a filter, the viewer 310perceives image 378 (A″). When the output of the display device 376 isperceived by a viewer 312 that is viewing the output of the displaydevice 376 through a demultiplexing filter 380, the viewer 312 perceivesimage 382 (H″). In one embodiment, the display device 376 is configuredto implement the multiplexing functional block 374. In alternativeembodiments, the display device 376 does not include the multiplexingfunctional block 374 and the input to the display device 376 is alreadymultiplexed. In some embodiments, the display device 376 comprises afilm projector, such as a film projector used in a cinema or home movietheater. In alternative embodiments, the display device 376 may beimplemented as a media player that is configured to playback an opticaldisc. For example, the media player may be a Digital Versatile Disc(DVD) player, Blu-ray player, HD-DVD player, or other type of mediaplayer. In still further embodiments, the display device 376 may beimplemented as a media player that is configured to playback data files.For example, the media player may comprise a software digital videomedia player.

As shown, images 378 and 358 are compared via the compare block 368,where the output of the compare block 368 is input into the processingfunctional block 364. Similarly, images 382 and 362 are compared via thecompare block 366, where the output of the compare block 366 is inputinto the processing functional block 364. Thus, the processingfunctional block 364, in the embodiment shown, generate the images 370and 372 based on four different inputs. In one embodiment, image 378 isidentical to image 354. In another embodiment, image 378 is identical toimage 358. In yet another embodiment, image 378 is not identical toeither image 354 or image 358. In these embodiments, image 378 may,however, be perceived as image 354 and/or image 358. For example, thedifference between image 378 and image 354 may be imperceptible to aperson, or the difference may be very slight yet perceptible.

Adding the feedback via the compare blocks 366 and 368 allows the system350 to compensate for calibration, drift, or errors, and/or to makeadjustments that are needed to compensate for the imperfectionsintroduced based on the limitations of the display device, theprocessing block 364, the multiplexor 374, and/or the demultiplexor 380.In one embodiment, the compare functional block 368 compares the images358 and 378 and generates data representing the difference between thetwo images 358, 378. A similar operation is performed by the compareblock 366 to generate a difference between the images 362, 382. Theprocessing block 364 takes these differences as inputs so as tocalibrate the processing operation performed by the processingfunctional block 364. For example, the output images that are outputfrom the processing functional block 364, when multiplexed with oneanother and displayed, (and potentially demultiplexed), are perceived asthe image 378 that is substantially similar to image 358. Similarly,image 382 is perceived as substantially similar to image 362. In thisfashion, imperfections in the physical world can be compensated for sothat the images 378 and 382 are perceived in the manner describedherein.

In some embodiments, the compare block 366 and/or 368 may work inreal-time, generating calibration feedback continuously as system 350generates and/or displays various images. In other embodiments, resultsof compare block 366 and/or 368 may be generated in advance by analysisand measurement of the characteristics of system 350, and processingblock 364 may operate using these precomputed results. In someembodiments, the use of physical display devices 356, 360 may not berequired to accurately predict images 358 (A′) and 362 (H′), and, thus,the compare blocks 366 and/or 368 can use predicted images rather thatactual displayed images. In other embodiments, the compare blocks 366and/or 368 operate directly using images 354 (spectrum A) and image 352(spectrum H), respectively, and display devices 356 and/or 360 areomitted from the system 350.

In some embodiments, as shown, images 370, 372, and the output of themultiplexor 374 may be stored in a memory 384 for future display.Additionally, as should be understood by those having ordinary skill inthe art, any of the images and/or inputs and/or outputs shown in FIG. 3Bmay also be stored in the memory 384. The memory 384 may be a server, adatabase, a RAM, a CD-ROM, a DVD-ROM, or any other type of technicallyfeasible memory and/or storage unit. Additionally, images stored inmemory 384 may be later reintroduced into system 350 for display to theviewers.

In some embodiments, a user identification system may be incorporated inthe system 350 so that different hidden images can be perceived bydifferent users. For example, the system 350 may be implemented in atheme park setting, where images are displayed on a wall. As describedabove, each person that looks at the wall without any specialized filter(i.e., in an ambient setting), perceives image 378. In contrast, personsthat look through specialized filters perceive a hidden image. Byincorporating the user identification system into the system 350,additional personalization and customization is possible. For example,different visitors to the theme park may be given specialized glasses,where each set of glasses includes a different radio frequencyidentification (RFID) tag that can be read by an RFID reader. The RFIDtag may uniquely identify the individual. Accordingly, different inputimages 352, 354 may be provided as inputs to the system 350 based on theRFID tag included in the glasses. Thus, in one embodiment, an ambientviewer perceives image 378, while a first viewer with a first RFID tagincluded in filtered glasses perceives a first hidden image and a secondviewer with a second RFID tag included in filtered glasses perceives asecond hidden image. In another embodiment, an identified person mayperceive a message directed to them individually, including, but notlimited to personalized messages (e.g., “Hi, John Doe!”), character orpersonality tie ins, scripted walkthroughs, or other events. In anotherembodiment, the RFID tag does not uniquely identify the individual, butinstead categorizes the individual into one of two or more groups ofindividuals. Different hidden images may be perceived by members of eachof the two or more groups of individuals. As persons having ordinaryskill in the art would understand, the user identification systemdescribed herein is not limited to RFID identification, and may includeany technique for user identification including cell phoneidentification, credit card identification, or any other technicallyfeasible technique for identifying a user or a group of users.

In one embodiment, to implement the user identification system, anidentity identification system (e.g., RFID reader) determines that theidentity of an individual corresponds to a first hidden image. Based onthe identity, a first pair of images 352, 354 is selected and used asinput into the system 350, where image 352 is a first hidden image andimage 354 is an ambient image. The result of processing the images 352,354 in the first pair via process block 364 is a pair of images 370, 372that is multiplexed, where image 370 is a first display image and image372 is a first delta image. If, however, the identity identificationsystem determines that identity of an individual corresponds to a secondhidden image, then a second pair of images 352, 354 is selected and usedas input into the system 350. Image 352 in the second pair of imagecorresponds to a second image and image 354 in the second paircorresponds to the ambient image. In one embodiment, the ambient imagein the first pair is the same as the ambient image in the second pair.The result of processing the images 352, 354 in the second pair viaprocess block 364 is a pair of images 370, 372 that is multiplexed,where image 370 is a second display image and image 372 is a seconddelta image. The pair of images 370, 372 that is output from theprocessing block is different when using the second pair of images asinput when compared to using the first pair of images as input.Accordingly, an ambient observer would perceive image 378 irrespectiveof which pair of images is used as input to the system 350. A personthat views the first multiplexed result using a filter that blocks thefirst delta image perceives the first hidden image; whereas, a personthat views the second multiplexed result using a filter that blocks thesecond delta image perceives the second hidden image.

FIG. 4A is a conceptual diagram illustrating a system 400 where timedivision multiplexing is used to display the display image 206,according to one embodiment of the invention. As shown, the system 400includes a projector 402 and a screen 404. The projector 402 projectsimages onto the screen 404. The images projected by the projector 402alternate between being images H and images Δ. In one embodiment images406A, 406B, and 406C are each repeated displays of image H. Similarly,images 408A, 408B, and 408C are each repeated displays of image Δ. Insome embodiments, images 406A, 406B and 406C are consecutive frames ofan animation or movie, while similarly, images 408A, 408B and 406C areconsecutive frames of an animation or movie. As described above, whenspectrum H at various image locations in the displayed image is combinedwith the corresponding spectrum Δ, the result is spectrum A. Thiscombination results in the display image 206 that is displayed on thescreen 404. As also described above, an ambient viewer, who is notviewing the screen 404 through any type of filter, perceives the displayimage 206. A viewer that views the screen 404 through a filter thatblocks spectrum Δ sees an image derived from one or more of images 406A,406B, 406C (including spectrum H). Again, as described above, both image206 (including spectrum A) and the image derived from one or more ofimages 406A, 406B, 406C (including spectrum H) are target images thatare intended to be perceived.

FIG. 4B is a conceptual diagram illustrating three images 458, 460, 456configured for steganographic display and creation of a fourth image454, according to one embodiment of the invention. As described above inFIG. 2, the image 454 (ambient image) and the image 452 (hidden image)may be input images. By calculating the difference, at each imagelocation, between image 454 and image 452, image 456 is determined(delta image), as illustrated in equation 462. Some embodiments of theinvention display the hidden image 452, which is meant to be seen bysome viewers, and the non-target delta image 456, which is not meant tobe seen. However, some techniques for image display technology used fordisplaying images 452 and/or 456 may have certain intrinsic flaws thatmake it possible for ambient viewers to momentarily see all or part ofthe hidden image 452. For example, the human physical behavior ofsaccading can cause the ambient viewer to momentarily perceive a singleimage that is presented in a time-division multiplexing display.Saccading makes it possible for an ambient viewer to unintentionallyperceive the hidden image 452, even without the use of time-divisionfilters.

To prevent this accidental exposure of the hidden image 452, anembodiment of the invention can divide the hidden image 452 into twoimages: an alpha image 458 and a beta image 460, as shown in FIG. 4B andillustrated in equation 464. In some embodiments, both the alpha image458 and the beta image 460 are non-target images, and individually arenot recognizable or perceivable images. For any given image location,when the spectrum of the alpha image 458 is combined with the spectrumof the beta image 460, the result is the hidden image 452.

Similarly, as described above, the delta image 456 may be computed thatis the difference between the hidden image 452 and the ambient image454. Accordingly, in this embodiment, the display system is configuredto display all three of the alpha image 458, the beta image 460, and thedelta image 456. As shown, a viewer that views the display image withouta filter views the combination of alpha image 458, beta image 460 anddelta image 456, and thereby perceives the target ambient image 452, asillustrated in equation 466. A viewer that views the displayed image 454using a filter that blocks the delta image 456, views the combination ofthe alpha image 458 and the beta image 460, and, thus, perceives thetarget hidden image 452, as illustrated in equation 468. When a viewerthat does not use a filter perceives all or part of the alpha image 458and/or the beta image 460 individually, the viewer does not perceive thehidden image 452, and the existence and content of the hidden image 452remains unexposed to the viewer.

FIG. 5 is a conceptual diagram illustrating three images 502, 504, 506configured for steganographic image display of a fourth image 508,according to one embodiment of the invention. As shown, images 502, 504,and 506 may be combined together to achieve image 508. Image 502, alsoreferred to herein as a first hidden image, includes image locationscontaining spectrum H1, image 504, also referred to herein as a secondhidden image, includes image locations containing spectrum H2, and image506 includes image locations containing spectrum Δ. As shown in equation510, when spectrum H2 is combined with spectrum H1 and spectrum Δ, theresult is spectrum A. A viewer that views the display image without afilter would perceive image 508, also referred to herein as an “ambientimage,” which is associated with spectrum A.

As shown in equation 512, excluding spectrum H2 and spectrum Δ fromspectrum (H1+H2+Δ) results in spectrum H1. In one embodiment, spectrum(H1+H2+Δ) is equivalent to spectrum A. A viewer that views the displayimage through a filter that blocks both spectrum H2 and spectrum Δperceives hidden image 502, which is associated with spectrum H1.

Similarly, as shown in equation 514, excluding spectrum H1 and spectrumΔ from spectrum (H1+H2+Δ) results in spectrum H2. A viewer that viewsthe display image through a filter that blocks both spectrum H1 andspectrum Δ perceives hidden image 504, which is associated with spectrumH2.

Each of images 502, 504, and 508 are target images that are intended tobe perceived. Image 506, on the other hand, is a non-target image thatis not intended to be perceived. As described herein, three differentviewers can see three completely different images by viewing the samedisplay image, where one viewer is viewing the display image without afilter and each of the other two viewers is using a different filter.

FIG. 6 is a conceptual diagram of a display image 618 viewed by threeviewers 602, 604, 606, according to one embodiment of the invention. Asshown, viewer 602 is viewing the display image 618 with no filter,viewer 604 is viewing the display image 618 through filter 608, andviewer 606 is viewing the display image 618 through filter 610. Filter608 is a “not(Δ), not(H2) filter” that does not pass spectra Δ nor H2,resulting in only spectrum H1 passing through. Filter 610 is a “not(Δ),not(H1) filter” that does not pass spectra Δ nor H1, passing onlyspectrum H2.

The display image 618 is composed of three separate images whose imagelocations include spectra H1, H2, and Δ, respectively. The viewer 602perceives view 612 that includes the combination of spectra H1, H2, andΔ, which results in spectrum A. The viewer 604 perceives view 614 thatincludes only spectrum H1. The viewer 606 perceives view 616 thatincludes only spectrum H2. Again, each viewer 602, 604, 606 perceives adifferent view 612, 614, 616, respectively, when looking at the displayimage 618. As persons having ordinary skill in the art would understand,any number of different images can be combined with one another and witha “Δ image” to achieve a display image, where four or more differentviewers can perceive the display image differently and where each of thefour or more images is a target image.

FIG. 7 is a flow diagram of method steps for generating a display image,according to one embodiment of the invention. Persons skilled in the artwill understand that, even though the method 700 is described inconjunction with the systems of FIGS. 1-6, any system configured toperform the method steps, in any order, is within the scope ofembodiments of the invention.

As shown, the method 700 begins at step 702, where a softwareapplication receives a first target image. The software application maybe stored in the system memory 104 and executed by the CPU 102 and/orthe display processor 112. In one embodiment, the software applicationis a rendering application executed by the CPU 102.

In some embodiments, the first target image is an image that is intendedto be perceived by a viewer. In one embodiment, the first target imageis a single still image. In alternative embodiments, the first targetimage is frame of a video or animation sequence.

At step 704, the software application receives a second target image. Insome embodiments, the second target image is also an image that isintended to be perceived by a viewer. In one embodiment, the secondtarget image is a single still image. In alternative embodiments, thesecond target image is frame of a video or animation sequence. In someembodiments, the second target image is completely different from thefirst target image. In other embodiments, the second target image issimilar to the first target image. For example, the second target imagemay represent a subset of the objects that are represented by the firsttarget image, or vice versa.

At step 706, the software application computes a delta image and asecond display image, where multiplexing the second display image withthe delta image achieves the first target image. As described herein,various techniques may be used to perform the multiplexing operationinvolving the second display image and the delta image. For example, themultiplexing operator may be, without limitation, a chromaticmultiplexing operator, a temporal multiplexing operator, a spatialmultiplexing operator, a multiplexing summing operator, a polarizationmultiplexing operator, or any other technically feasible multiplexingoperator. The implementation of step 706 may be different depending onwhat type of multiplexing operator is used to multiplex the seconddisplay image with the delta image.

At step 708, the software application displays and/or stores data thatrepresents the second display image multiplexed with the delta image.The data generated may be stored in a memory or displayed by a displaydevice. Again, the implementation of step 708 may be different dependingon what type of multiplexing operator is used to multiplex the seconddisplay image with the delta image. In one embodiment, when the datathat is generated is displayed on a display device, a viewer that is notusing a filter perceives the first target image (e.g., in an ambientsetting). A viewer that is viewing the generated data through a filterthat blocks the delta image perceives the second target image.

FIG. 8 is a flow diagram illustrating how using a filter causes a viewerto perceive the target image differently, according to one embodiment ofthe invention. Persons skilled in the art will understand that, eventhough the method 800 is described in conjunction with the systems ofFIGS. 1-6, any system configured to perform the method steps, in anyorder, is within the scope of embodiments of the invention.

As shown, the method 800 begins at step 802, where a softwareapplication causes a delta image multiplexed with a second display imageto be displayed. In one embodiment, the image(s) that are displayed atstep 804 are generated at step 708, described in FIG. 7.

Referring back to FIG. 8, at step 804, if the viewer is looking at thedisplay image without a filter, then the method 800 proceeds to step806, where the viewer sees the combination of the delta image and thesecond target image, and then immediately proceeds to step 807, wherethe viewer perceives the first target image

If, at step 804, the viewer is looking at the display image through afilter, then the method 800 proceeds to step 808. At step 808, the deltaimage is filtered out from the display image by the filter. Thus, atstep 810, the viewer perceives the second target image.

Again, as described herein, both the first target image and the secondtarget image are images that are intended to be perceived. In someembodiments, since the delta image provides the difference between thefirst target image and the second target image, the delta image islikely to be a seemingly noisy and/or undesirable image that is notintended to be viewed.

Additionally, embodiments of the invention are not limited to two orthree different target images that can be displayed simultaneously.Embodiments of the invention apply equally well to any number ofdifferent “target” images that can be combined with a delta image togenerate another target image.

Advantageously, embodiments of the invention provide a technique toobscure an image from an ambient observer, while still providing theambient observer with a target image that is meant to be perceived.Thus, the ambient observer would not be able to detect that an image isbeing hidden within the display image, or obscured by the display imagethat the ambient observer perceives.

Various embodiments of the invention may be implemented as a programproduct for use with a computer system. The program(s) of the programproduct define functions of the embodiments (including the methodsdescribed herein) and can be contained on a variety of computer-readablestorage media. Illustrative computer-readable storage media include, butare not limited to: (i) non-writable storage media (e.g., read-onlymemory devices within a computer such as CD-ROM disks readable by aCD-ROM drive, flash memory, ROM chips or any type of solid-statenon-volatile semiconductor memory) on which information is permanentlystored; and (ii) writable storage media (e.g., floppy disks within adiskette drive or hard-disk drive or any type of solid-staterandom-access semiconductor memory) on which alterable information isstored.

The invention has been described above with reference to specificembodiments and numerous specific details are set forth to provide amore thorough understanding of the invention. Persons skilled in theart, however, will understand that various modifications and changes maybe made thereto without departing from the broader spirit and scope ofthe invention. The foregoing description and drawings are, accordingly,to be regarded in an illustrative rather than a restrictive sense.

What is claimed is:
 1. A computer-implemented method for generatingimages, the method comprising: receiving a first target image comprisinga first human-perceptible image; receiving a second target image,dissimilar from the first target image, comprising a secondhuman-perceptible image; subtracting the second target image from thefirst target image, at each image location, to generate a first displayimage; generating a second display image that is perceivable as thesecond target image; splitting the second display image into a thirddisplay image and a fourth display image, wherein neither the thirddisplay image nor the fourth display image is recognizable as the secondtarget image, and multiplexing the first display image with the thirddisplay image and the fourth display image to generate a fifth displayimage that is perceivable as the first target image when viewed in anambient setting; wherein the fifth display image, when viewed via avisual aid, is perceivable as the second target image.
 2. The method ofclaim 1, wherein the second display image is similar to the secondtarget image.
 3. The method of claim 1, wherein the second display imagecorresponds to the second target image but is not similar to the secondtarget image.
 4. The method of claim 1, further comprising displayingthe first display image multiplexed with the second display image on adisplay device, or storing the first display image multiplexed with thesecond display image in a memory.
 5. The method of claim 4, whereindisplaying comprises projecting the first display image and the seconddisplay image in an alternating pattern.
 6. The method of claim 5,wherein the first display image multiplexed with the second displayimage, when viewed by the person though a shutter that blocks onedisplay image each time that image is projected, is perceived as thesecond target image to the person.
 7. The method of claim 4, wherein thefirst display image multiplexed with the second display image, whenviewed by the person though a filter that blocks one display image, isperceived as the second target image.
 8. The method of claim 1, whereinmultiplexing the first display image with the second display imagecomprises performing time division multiplexing.
 9. The method of claim1, wherein multiplexing the first display image with the second displayimage comprises performing wavelength division multiplexing, phasedivision multiplexing, polarization multiplexing, and/or chromaticmultiplexing.
 10. The method of claim 1, wherein computing the firstdisplay image and the second display image is further based in part on adifference between a first compare image that is perceived when thefirst display image multiplexed with the second display image isdisplayed on a first display device and a second compare image that isperceived when the first target image is displayed on a second displaydevice.
 11. The method of claim 1, wherein computing the first displayimage and the second display image is further based in part on adifference between a first compare image that is perceived when thefirst display image multiplexed with the second display image is viewedby the person though a filter that blocks one display image and a secondcompare image that is perceived when the second target image isdisplayed on a second display device.
 12. The method of claim 1, whereinthe first target image represents a scene that is a subset of a scenerepresented by the second target image.
 13. The method of claim 1,wherein the second target image represents a scene that is a subset of ascene represented by the first target image.
 14. The method of claim 1,wherein the first target image is visually unrelated to the secondtarget image.
 15. A computer-readable storage medium storinginstructions that, when executed by a processor, cause a computer systemto generate images, by performing the steps of: receiving a first targetimage comprising a first human-perceptible image; receiving a secondtarget image, dissimilar from the first target image, comprising asecond human-perceptible image; subtracting the second target image fromthe first target image, at each image location, to generate a firstdisplay image; generating a second display image that is perceivable asthe second target image; splitting the second display image into a thirddisplay image and a fourth display image, wherein neither the thirddisplay image nor the fourth display image is recognizable as the secondtarget image, and multiplexing the first display image with the fourthdisplay image and the fifth display image to generate a fifth displayimage that is perceivable as the first target image when viewed in anambient setting; wherein the fifth display image, when viewed via thevisual aid, is perceivable as the second target image.
 16. Thecomputer-readable storage medium of claim 15, wherein the second displayimage is similar to the second target image.
 17. The computer-readablestorage medium of claim 15, wherein the second display image correspondsto the second target image but is not similar to the second targetimage.
 18. The computer-readable storage medium of claim 15, wherein thefirst display image multiplexed with the second display image, whenviewed by the person though a filter that blocks one display image, isperceived as the second target image.
 19. The computer-readable storagemedium of claim 15, wherein multiplexing the first display image withthe second display image comprises performing time divisionmultiplexing, wavelength division multiplexing, phase divisionmultiplexing, polarization multiplexing, and/or chromatic multiplexing.20. The computer-readable storage medium of claim 15, wherein computingthe first display image and the second display image is further based inpart on a difference between a first compare image that is perceivedwhen the first display image multiplexed with the second display imageis displayed on a first display device and a second compare image thatis perceived when the first target image is displayed on a seconddisplay device.
 21. The computer-readable storage medium of claim 15,wherein computing the first display image and the second display imageis further based in part on a difference between a first compare imagethat is perceived when the first display image multiplexed with thesecond display image is viewed by the person though a filter that blocksone display image and a second compare image that is perceived when thesecond target image is displayed on a second display device.
 22. Asystem, comprising: a device configured to: cause a first display image,that is perceivable as a first human-perceptible target image whenviewed in an ambient setting, to be displayed, wherein the first displayimage is generated by multiplexing a second display image with a thirddisplay image and a fourth display image, wherein the third displayimage and the fourth display image are generated by splitting a fifthdisplay image into the third display image and the fourth display image;wherein the fifth display image is generated by subtracting a secondhuman-perceptible target image, dissimilar from the firsthuman-perceptible target image, from the first human-perceptible targetimage, at each image location, wherein the second display image isperceivable as the second human-perceptible target image, wherein thefirst display image, when viewed via a visual aid, is perceivable as thesecond human-perceptible target image.
 23. The system of claim 22,wherein the fifth display image is similar to the second target image.24. The system of claim 22, wherein the fifth display image correspondsto the second target image but is not similar to the second targetimage.
 25. The system of claim 22, wherein the device is furtherconfigured to multiplex the second display image with the fifth displayimage.
 26. The system of claim 22, further comprising a multiplexingdevice configured to multiplex the second display image with the fifthdisplay image.
 27. The system of claim 22, wherein the second displayimage multiplexed with the fifth display image, when viewed by theperson though a filter that blocks one display image, is perceived asthe second target image.
 28. The system of claim 22, wherein the seconddisplay image is multiplexed with the third display image and the fourthdisplay image using time division multiplexing, wavelength divisionmultiplexing, phase division multiplexing, polarization multiplexing,and/or chromatic multiplexing.
 29. The system of claim 22, wherein thedevice comprises a film projector.
 30. The system of claim 22, whereinthe device comprises a media player configured to playback an opticaldisc or digital data on a display device.
 31. The system of claim 30,wherein the display device comprises a television or computer display ordigital projector.
 32. A computer-implemented method for generatingimages, the method comprising: receiving a first target image comprisinga first human-perceptible image; receiving a second target image,dissimilar from the first target image, comprising a secondhuman-perceptible image; receiving a third target image, dissimilar fromthe first target image, comprising a third human-perceptible image;subtracting the second target image and the third target image from thefirst target image, at each image location, to generate a first displayimage; generating a second display image that is perceivable as thesecond target image; generating a third display image that isperceivable as the third target image; splitting the third display imageinto a fourth display image and a fifth display image, wherein neitherthe fourth display image nor the fifth display image is recognizable asthe third target image; and multiplexing the first display image withthe second display image, the fourth display image, and the fifthdisplay image to generate a sixth display image that is perceivable asthe first target image when viewed in an ambient setting; wherein thesixth display image, when viewed via a first visual aid, is perceivableas the second target image, wherein the sixth display image, when viewedvia a second visual aid, is perceivable as the third target image. 33.The method of claim 32, wherein the second display image is similar tothe second target image, and wherein the third display image is similarto the third target image.
 34. The method of claim 32, wherein thesecond display image corresponds to the second target image but is notsimilar to the second target image, or the third display imagecorresponds to the third target image but is not similar to the thirdtarget image.
 35. The method of claim 32, further comprising displayingthe first display image multiplexed with the second display image, thefourth display image, and the fifth display image on a display device,or storing the first display image multiplexed with the second displayimage and the third display image in a memory.
 36. The method of claim32, wherein the first display image multiplexed with the second displayimage, the fourth display image, and the fifth display image, whenviewed by the person though a filter that blocks the first display imageand the third display image, is perceived as the second target image.37. A computer-implemented method for generating images, the methodcomprising: receiving a first target image comprising a firsthuman-perceptible image; receiving a second target image, dissimilarfrom the first target image, comprising a second human-perceptibleimage; subtracting the second target image from the first target image,at each image location, to generate a first display image; generating asecond display image that is perceivable as the second target image; andmultiplexing the first display image with the second display image togenerate a third display image that is perceivable as the first targetimage when viewed in an ambient setting; wherein the third displayimage, when viewed via a visual aid, is perceivable as the second targetimage, wherein computing the first display image and the second displayimage is further based in part on a difference between a first compareimage that is perceived when the first display image multiplexed withthe second display image is displayed on a first display device and asecond compare image that is perceived when the first target image isdisplayed on a second display device.